This invention relates to an electromagnetic type fuel injection valve, and in particular, to an electromagnetic type fuel injection valve suitable for use as a fuel injection valve in an automotive fuel supply system.
As disclosed, for example, in Japanese Patent Examined Publication No. 56-11071, this type of electromagnetic fuel injection valve comprises a stator iron core made of a magnetic material and including a flange section, a casing made of a magnetic material, an electromagnetic coil surrounded by this casing, a moving body, and a needle valve. When an electric current is passed through the electromagnetic coil, a magnetic circuit is formed, the electromagnetic force thus formed urging the moving body to open and close the needle valve. The moving body, that is, a principal component, is composed of an armature, a rod, and a valve body. The valve body is required to have abrasion resistance and corrosion resistance since it hits against a valve guide in the fuel pasage. In view of this, the valve body is normally made of a high-carbon (C) and high-chrome (Cr) martensite base stainless steel of JIS SUS440C class, which is hardened and tempered to give it a Rockwell hardness of around Hrc60. Since the rod of the moving body hits against a stopper, the rod also needs to have abrasion resistance and corrosion resistance, so it is made of a material of the same type as the valve body. The valve body and the rod are connected to each other by means of electric resistance welding, laser welding, plasma welding, electron beam welding, etc.
Since the armature of the moving body forms a magnetic circuit together with the stator iron core and the casing, its material is a low-carbon and high-chrome electromagnetic stainless steel containing silicon which is of the same type as is used for the stator iron core and the casing. That is, the armature is normally worked into a ring-like configuration by means of a lathe, and is annealed at a temperature in the range of 900.degree. to 1100.degree. C. to remove therefrom internal strain and internal residual stress, its crystal grain size being enlarged so that it possesses the desired electromagnetic properties. Afterwards, it is connected to the rod by means of laser welding, electron beam welding, force fitting, press fitting or the like. This connecting operation results in considerable generation of strain and residual stress in the armature, thereby causing a deterioration in the magnetic properties (coercive force and magnetic flux density). On the other hand, the exciting force that serves as the absorbing force of the armature creates a leakage magnetic path leading to the casing through the rod which constitutes the needle valve and the valve guide which constitutes the nozzle body. Accordingly, the rod is subjected to absorption around and suffers abrasion while moving in the vertical direction. Hence the absorbing force needs to be reduced and the abrasion resistance of the rod enhanced. In a case where the armature is connected to the rod by means of press fitting as disclosed in Japanese Patent Examined Publication No. 56-11071, the joint section is inevitably made long so that the predetermined degree of binding strength can be obtained.